Biobased ester products typically consist of long-chain fatty acid alkyl (methyl, ethyl, propyl, or butyl) esters derived from triglycerides present in vegetable oil, animal fat, or plant lipids by transesterification type reactions. For example, biodiesel is an environmentally friendly fuel, non-toxic, and identified as biodegradable. Due to global climate changes and the decline in world crude oil production along with the rising prices of petroleum products, biodiesel has received considerable attention and may offer a promising alternate energy resource.
Currently, biodiesel is commercially produced from edible vegetable oils such as palm oil, corn oil, coconut oil, sunflower oil, and soybean oil. The fatty acid triglycerides of those oils are converted to the respective alkyl ester (the biodiesel) and glycerin by aqueous based acid-catalyzed or base-catalyzed transesterification in the presence of an excess amount of methanol or ethanol. Biodiesel can then be separated from glycerin and the glycerin by-product can be used to make soap. In addition, the alkyl ester products may be hydrogenated to provide various alcohols which may then serve to supply a variety of industrial end products.
Examples of acid catalysts that have been used for the transesterification reactions include sulfuric, sulfonic, phosphoric, and hydrochloric acid. The presence of water typically has a negative influence on the effectiveness of such acid catalysts, which water must be removed in order to maintain the catalytic efficiency. As a result; base catalysts are usually preferred compared to acid catalysts because of relatively higher conversion rates and relatively lower process temperatures compared to the acid-catalyzed transesterification process. Base catalysts include metal hydroxides, metal alkoxides, or alkaline-earth oxides.
As a consequence, current industrial transesterification procedures are performed in batch stirred tank reactors at temperatures ranging from 60 to 200° C. using homogeneous base catalysts in water solution. However, the use of homogeneous base catalysis typically requires neutralization and separation from the final reaction mixture and relatively high solvent consumption. Thus, the additional production costs of the bioderived ester are higher and not as competitive with the production costs of natural and petroleum-derived esters.